The invention relates to an electron spin resonator, and more particularly to a cavity resonator for an electron spin resonator.
For electron spin resonators, a sensitivity in detection is one of the most important factors. In the prior arts, it seems to be general for improvement in the sensitivity in detection to utilize a superimposition of a modulation magnetic field with a frequency on a polarizing static magnetic field for an amplification and subsequent detection of only a signal component which appears in the electron spin resonance. It is desirable to suppress a low frequency noise due to a signal amplification. This requires the modulation magnetic field to have a possible high frequency but in a responsible range of a microwave cavity resonator. For example, a 100 kHz frequency is generally available as the frequency of the modulation magnetic field. The modulation magnetic field is caused by applying a high frequency current to a modulation coil which is arranged either in an exterior or an interior of the cavity resonator. The former is an external modulation system and the latter is an internal modulation system.
The above two types of the cavity resonator are subject to the following problems respectively which will be described in detail.
Problems with the internal modulation type cavity resonator will be investigated in which the frequency of the modulation magnetic field is 100 kHz. The Q factor of the cavity resonator is associated with the sensitivity of detection of the electron spin resonator. Of course, it is desirable that the Q factor have a possible high value. In the internal modulation type cavity resonator, the arrangement of the modulation coil within the cavity resonator, however, causes the Q factor to be lowered. The suppression of lowering the Q factor requires the modulation coil to receive restrictions in the number of turns and in the turn ratio. Physically, the number of turns of the modulation coil is restricted down to one or two turns. That is why it is difficult to obtain a high efficiency in generation Of the modulation magnetic field by such a modulation coil. The generation of a necessary modulation magnetic field for causing the electron spin resonance requires applying an extensive large modulation current to such the modulation coil having less number of turns. The accomplishment of the electron spin resonance requires such a precise operation as to suppress a leakage of microwave that appears where wires of the modulation coil penetrate the wall of the cavity resonator. The extensive large modulation current flowing in the wire of the modulation coil causes a rise of temperature of the wire of the modulation coil at a penetrated portion of the cavity resonator wall and thereby a looseness at the penetrated portion often appears.
On the other hand, although the external modulation type cavity resonator is free from the above mentioned problems, that it has the following problems. In this case, as the modulation coil is arranged at the exterior of the cavity resonator, the modulation magnetic field is required to penetrate the wall of the cavity resonator. Since the wall of the cavity resonator is made of a conductive material, the modulation magnetic field or alternating magnetic field penetrating the cavity resonator wall causes an eddy current to appear around the penetrated portion of the cavity resonator so as to suppress the magnetic field. As a result, the eddy current prevents the transmission of the modulation magnetic field through the wail of the cavity resonator, resulting in a considerable reduction of the transmittivity of the modulation magnetic field through the wall of the cavity resonator. When a TE.sub.102 rectangular cavity resonator having a side wall thickness of approximately 1.2 mm is used, the intensity of the original modulation magnetic field of 100 kHz, which appears at the exterior of the cavity resonator, is considerably attenuated down to 10.sup.-4 times thereof or less. So it is difficult to secure a necessary intensity of the modulation magnetic field at the interior of the cavity resonator. Such an insufficient intensity of the modulation magnetic field results in an inferiority in the sensitivity of the cavity resonator. Even if not so, it provides a restriction to the improvement in the sensitivity of the cavity resonator. Securing a necessary intensity of the modulation magnetic field at the interior of the cavity resonator requires further generating an extensive large modulation magnetic field at the exterior of the cavity resonator, which is of course undesirable.
The external modulation type cavity resonator is further subject to the following problem. As described above, as the modulation coil is arranged at the exterior of the cavity resonator, the modulation magnetic field appearing at the modulation coil is transmitted through the wall of the cavity resonator into the interior thereof. The transmission of the modulation magnetic field through the cavity resonator wall generates an eddy current at the cavity resonator wall. The eddy current exhibits an interference with the static magnetic field applied to the cavity resonator. The interference between the eddy current and the static magnetic field causes the cavity resonator to exhibit an oscillation whose frequency is almost equal to the frequency of the modulation magnetic field. This provides a great deal of influence to the sensitivity of the cavity resonator. Namely, this results in a remarkable inferiority in the sensitivity of the cavity resonator.
To settle the above two problems with the external modulation type cavity resonator, it has been proposed to apply a low frequency modulation magnetic field to the cavity resonator, the reason of which is as follows. When the modulation magnetic field has a relatively high frequency, the eddy current caused by that has also a relatively high frequency. Such a high frequency eddy current at the wall of The cavity resonator shows the skin effect having a skin depth which is inversely proportional to the square root of the frequency of the eddy current. The frequency of the modulation magnetic field is almost equal to that of the eddy current. When the frequency of the modulation magnetic field becomes small, the skin depth of the skin effect of the eddy current becomes large. The frequency of the modulation magnetic field is so lowered that the skin depth of the skin effect of the eddy current becomes nearly equal to the thickness of the wall of the cavity resonator. This prevents the attenuation of the intensity of the modulation magnetic field when it penetrates the wall of the cavity resonator. For example, the frequency of the modulation magnetic field is required to be lowered down to about 10 Hz to 300 Hz for preventing the attenuation of the intensity of the modulation magnetic field. In this case, since the frequency of the modulation magnetic field is low, as described above the sensitivity of the cavity resonator is forced to suffer the inferior affection by the low frequency noise which appears in the signal amplifier.
To settle all of the above problems with the above both cavity resonator types, it is thus required to develop a quite novel cavity resonator for electron spin resonators which both is free from an extensive large modulation current and is able to secure a necessary intensity of the modulation magnetic field at the interior of the cavity resonator without undesirable oscillation of the cavity resonator for implementation of a high sensitive detection of the electron spin resonance.